Electron discharge tube with means for trapping positive ions



Nov. 17, 1959 L. M. FIELD ELEcTRoN DISCHARGE TUBE WITH MEANS Fox TRAPPING POSITIVE IONS med Aug. 2v, 195e .F/ELD M\N\N- United States Patent() ELECTRON :DISCHARGE TUBE WIT H lVIEANS FOR TRAPPING VPOSITIVEIONS Lester M. Field, Los Angeles, Calif., assignor to Sperry Rand Corporation, a corporation of Delaware .Application August 27,1956, sesamo'. 606,519

claims (ci. 31a- 16) This invention relates to improvements in electron discharge tubes such as traveling wave tubes wherein an electron beam .is projected at high velocity through an interaction space, and more particularly `to the prevention of cathode bombardment by positive ions which are formed in the operation of such tubes.

In the operation of' traveling wave tubes and similar devices, a beam of electrons is projected at high `velocity through a relatively long path such as that delined by the interior `of an elongated helix or simil-ar radio `frequency circuit element. Some of the electrons collide with resid-y ual gas molecules that unavoidably remain. in the tube after evacuation, creating ions. The positive ions, under the inuence of the same fields that are used to form and accelerate the electron beam tend to travel toward the `cathode from which the electrons are emitted, bombarding the cathode and destroying its emitting surface where they strike it.

This effect is not particularly troublesome in some types of tubes, because such tubes are designed to use a beam that converges from a relatively large cathode surface down toa relatively Vsmall diameter before itenters the interaction space. Since the ions. have masses Vof the orderof 2000 times the electron mass., or more, they are not readily deected from their initially linear paths by the electrostatic fields that make the electron beam converge. Thus the bombardment tends to be concentrated on the central part of the cathode which is in line with the small-diameter linear portionv of the electron beam. This central part of the cathode is onlya'small portion of the total cathode area, so its lossvis not serious.

However, in traveling -wave tubes that Iare specially designed for low noise, and particularly in low-noise tubes of the type employing velocity-jump `electron guns, it is necessary to use cathodes of relatively small active area to insure that the electrons are emitted at more uniform velocities. It is also necessary that the beam be collimated at the emitting surface. Thus the cathode surface is of approximately the same size as the beam, and usually is immersed in the same` uniform axial magnetic field that is used to maintain `the beam in focus as it passes through Ithe interaction region of the tube. As a result, the ion beam striking the cathode has approximately the same diameter `as the cathode andtends to damage substantially the entire cathode surface. For this reason most low-noise tubes heretofore have had unsatisfactorily short useful lives.

The principal object ot this invention is to provide means for minimizing or substantially preventing the above described cathode bombardment.

Another object is to provide cathode protective means that will introduce little or no disturbance in the focussing of lthe electron beam.

A further object is to provide simple and inexpensive means for accomplishing the foregoing objects, requiring but slight modification of existing tube structures and the addition of an auxiliary magnetic eld forming means.

Vice

Vlccording.; to the present invention, the electron beam islformed and projected initially at a small angle to the axis of its path Athrough the interaction space, and bent by means of a magnetic iield so as to enter and traverse the interaction space in `a normal manner. Positive ions that are formed in the interaction space, where the major part of the pathof the beam lies, accelerate in the interaction space to such an extent that they are unable to round the bend like the electrons, owing to theirmuch larger masses. Thus substantially all of the returning positive ions miss the cathode completely andare collected on other parts of the structure where they do no harm.

The invention will be described with reference to the accompanying drawing, wherein:

Fig. 1 is a longitudinal section of a traveling wave tube embodying the invention, and

Fig. 2 is a longitudinal section of a portion of a tube embodying a modiication of thestructure of Fig. l.

The traveling Wave tube shown in Fig. 1 includes an evacuated envelope 1 containing an electron gun 3, a

conductive helix 5, and a collector electrode 7. The ends of the helix 5 are coupled by impedance-matching transition means 8 and f9 lrespectively to input and output coaxial transmission lines 10 and 11 which extend through the Wall of the envelope 1 for connection to appropriate high frequency source and load means, not shown. An electromagnet 12 surrounds the envelope 1 for producing a uniform magnetic field directed along the longitudinal axis 13 of the helix 5 and immersing the entire operating structure, including the electron gun 3.

The electron gun 3 includes a cathode` 14 which is in the form of a thimble with an end face 15 coated with electron emissive material, and a plurality of electrodes 16,- 17 and 18 arranged to focus and accelerate a beam of electrons emitted from the surface 15. The electrodes 16, 17 and 18 may be designed as ydescribed in U.S. Patent No. 2,800,602,'issued on July 23, 1957, 4to Lester M. Field and Dean A. Watkins, although such designisnot an essential feature of the present invention. The means g for supporting and providing external connections -to the gun 3 may be of known `or conventional type, as shown in Fig. l. A DSC. source such as a battery 24 is connected to the cathode 14-and collector 7, and is provided with terminals connected to supply appropriate voltages to the gun elements. A current meter 26 may be included in the collector connection. The cathode 14 is provided with a heater arranged to be energized from a source such as a battery 28.

As indicated in the drawing, the axis of symmetry 1,9 of the electro-n gun 3 is disposed at a small angle with respect to the axis 13 ofthe helix 5,V intersecting it at a point 20 near the input end ofthe helix. An auxiliarymagnetic tield producing device `4 is disposed outside the envelope 1 in the vicinity of the electron gun 3. The device `4 may be a magnet winding similar to a deflection coil like those used for magnetic deflection of the beam in a cathode ray oscilloscope or a television picture tube. It is oriented with its central axis 21 extending transversely of the helix axis 13, and lying substantially in the plane dened by the -axes 13 and 19. A D.C. source such as a battery 23 is arranged to energize the electromagnets 4 and 12. The intensities of the magnetic lields produced by magnets 4 and 12 'may be regulated by means` of adjustable resistors 25 and 27 respectively.

In the-operation of the structure of Fig. 1, the electromagnet 12 provides a magnetic field directed along the axis'13 of the helix 5 for maintaining the electron beam in focus throughout its path within the helix, as in conventional traveling wave tubes. The auxiliary electromagnet 4 produces another magnetic field directed transversely of, and preferably approximately at a right angle to the axis' 13, but substantially parallel to the planel determined by the axes 13 and 19.

The strength of the main field is adjusted by means of resistor 27 to a value sufficient to maintain the beam in focus within the helix. The strength of the auxiliary eld is adjusted by means of the resistor 25 to a value such that the resultant of the main and auxiliary elds in the region of the gun 3 is directed along the gun axis 19. This adjustment may be made conveniently by observing the collector current meter 26 and varying the resistor 25 to obtain a maximum meter reading, indicating that most of the electrons emitted by the cathode are going through the gun electrodes 17, 18 and the helix 5 and are reaching the collector 7.

When the above described adjustments have been made, the gun 3 operates in the normal manner of an electrostatic gun immersed in an axial magnetic field, directing the electron beam along the axis 19. As the beam approaches the point 20, the strength of the auxiliary field tapers off to a negligible value. The resultant field at the beam curves in the region of the point 20, from the direction of the axis 19 to that of the axis 13. Since the electrons tend to follow the magnetic field lines, the beam also curves in the vicinity of the point 20, then continues along the axis 13. i

Positive ions are formed throughout the path of the electron beam, but principally inside the helix, where the longest part of the path lies and where the beam has its maximum velocity. These ions, being positively charged, tend to form a beam of their own traveling in the direction opposite to the electron flow. The ions arrive at the point 20 with various velocities along the axis 13, moving from right to left in Fig. 1. Those having sufficient velocity to damage the cathode, if they were to strike it, have so much momentum in the direction of the axis 13 that they are not deflected by the auxiliary field to a sufiicient extent to reach the cathode. Thus most of the returning ions are collected on one or more of the gun electrodes, for example electrode 16.

The magnetic field modifying means of Fig. 1 has substantially no deleterious effect on the focusing of the electron beam, because the bearnsees merely a field that is slightly bent in the vicinity of the point 20. It should be noted that the orientation of the auxiliary field is unlike that used for magnetic deflection of a cathode ray beam in an oscilloscope or television picture tube, being parallel to the plane of the axes of the beam before and after bending, rather than perpendicular to said plane.

A modification of the device of Fig. 1 is illustrated in Fig. 2, wherein a wedge shaped magnetic pole piece 31 is provided between the electron gun 3 and the interaction region 5. The pole piece 31 is preferably made of a magnetically permeableV material that is readily scalable to glass, such as fernico. The envelope 1 is sealed to one face of the member 31 as at 34. Another envelope portion or bulb 1', containing the electron gun assembly 3, is sealed to the other face of the pole piece.

The left and right hand faces of the piece 31 are perpendicular respectively to the axes 19 and 13. An aperture 33 is provided at the point 20 for the passage of the electron stream. An auxiliary magnet coil 12', similar to but shorter than the coil 12, surrounds the bulb portion 1' with its axis in alignment with the gun axis 19.

In the operation of the device of Fig. 2, the coils 12 and 12 are energized so as to provide fields of substantially equal strengths along their respective axes. Since the magnetic lines of force will enter the pole piece at right angles to the respective faces, the electron beam will encounter a more or less abrupt change in the direction of the magnetic field at the point 20. The effect is substantially the same as that of the curved field in the device of Fig. 1, bending the electron stream more than the returning ion stream.

The coils 12 and 12, may be` surrounded by hollow cylindrical sleeves of magnetic material if desired, to provide low reluctance return paths for the magnetic circuits. Suitably proportioned permanent magnets may be used instead of electromagnets, in accordance with known prior art practice. Since the pole piece 31 is or may be of electrically conductive material, it presents to the beam at the aperture 33 the interior surface of a hollow conductive cylinder, like the electrode 18 of Fig. 1. Therefore the pole piece itself may be used as one of the gun electrodes. If desirable, it may be provided with a cylindrical skirt 18', preferably of non-magnetic conductive material such as copper, to make the electrode longer than the thickness of the pole face at the aperture 33.

While the invention has been described in its preferred embodiment, it is to be understood that the words which have been used are words of description rather than of limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.

What is claimed is:

1. An electron discharge tube including radio frequency circuit means defining a cylindrical space that is substantially longer than its diameter and comprises an interaction region through which a beam of electrons is to be projected substantially concentrically with the longitudinal axis of said space, means comprising an electron gun adjacent one end of said circuit means and including a substantially disc-shaped cathode and cooperating focussing electrodes for producing and projecting a beam of electrons toward said end of said circuit means along a second axis that intersects said axis of said interaction space at a point near said end and at an angle such that the axial projection of said interaction space lies outside the disc of said cathode, means for producing and maintaining an axial magnetic field throughout said interaction space, and means for producing and maintaining a second magnetic field substantially throughout the region between said cathode and said end of said circuit means in a direction parallel to said second axis, whereby an electron beam produced by said electron gun is directed toward and through said interaction space, and positive ions that are produced in said interaction space are prevented from reaching said cathode.

2. The invention set forth lin claim l, including a Wedge-shaped body of paramagnetic material disposed between said cathode and said interaction region, having a surface facing said interaction region and perpendicular to the axis thereof and another surface facing said cathode and substantially perpendicular to the line between said cathode and said point, said body having an aperture for the passage of said beam therethrough, said body forming a pole piece for said field-producing means.

3. The invention set forth in claim 1, including an anode electrode disposed between said cathode and said interaction region, said anode electrode comprising a wedge-shaped body of paramagnetic material having a surface facing said interaction region and perpendicular to the axis thereof and another surface facing said cathode and substantially perpendicular to the line between said cathode and said point, said electrode having an aperture for the passage of said beam therethrough, said anode electrode forming a pole piece for said field-producing means.

4. An electron discharge tube including means defining a hollow cylindrical interaction region open at its ends and having a longitudinal axis, an electron gun having a cathode for producing a beam of electrons, said cathode lying outside the axis projection of said interaction region to prevent bombardment of said cathode by positive ions formed inV said region,said electron gun being disposed to direct said electron beam toward a point on said longitudinal axis near the end of said interaction region adjacent said gun, means for producing a magnetic field parallel to the axis of said interaction space and extending throughout said space and the region between said cathode and said interaction region, and means for producing a further magnetic field component transverse to said iirst mentioned field and parallel to the plane deiined by said axis and the line from said cathode to said point on said axis, said further iield modifying said iirst mentioned iield to produce a resultant magnetic field directed along said line from said cathode to said point.

5. An electron discharge device wherein a beam of elec trons is to be directed along a predetermined axis, and wherein said beam produces positive ions that tend to iiow in the direction opposite to the oW of electrons in said beam, said device including means for producing a uniform magnetic iield directed along said axis and extending throughout the path of said beam along said axis, an electron gun having a cathode and cooperating electrodes for producing the electron beam and projecting said beam along a second axis oblique to said first axis toward a point on said first axis, the length of said path along said second axis being substantially less than the length of said path along said first axis, and means for producing a second uniform magnetic tield directed along said second axis and extending substantially throughout the path of said beam between said cathode and said point to cause said beam to follow said second axis before reaching said point and to follow said rst axis beyond said point, the angle between said rst and second axes being such that the rearward projection of the part of said beam lying on said rst axis lies substantially outside said cathode, whereby positive ions produced in said part of said beam are prevented from bombarding said cathode.

6. A travelling Wave tube including a wave propagating structure deiining an extended interaction region open at its ends and having a longitudinal axis, means for producing a uniform magnetic field throughout said region directed along said axis, an electron gun having a cathode for producing a beam of electrons, said cathode lying outside the axial projection of said interaction region to prevent bombardment of said cathode by positive ions formed in said region, said electron gun being disposed to direct said electron beam toward a point on said longitudinai axis near the end of said interaction region adjacent said gun, and means for producing a second uniform magnetic iield directed along the line between said cathode and said point and extending substantially throughout the path of said beam between said cathode and said point. 

